Photosensitive elements which can be used in image-reproduction processes are well-known in the graphics arts industry. Such elements are usually exposed to actinic radiation through an image-bearing transparency, such as a color separation transparency, to produce an image which is either a positive or negative with respect to the transparency used.
Such photosensitive elements are widely used in off-press color proofing to simulate the images produced by printing. In a surprint proof, all of the colored images are superposed, by, for example, multiple exposure, lamination, or transfer, on a single support. The colored images cannot be separated and viewed individually. In an overlay proof, the colored images can be viewed individually.
Positive-working photopolymerizable elements are described in Chu and Cohen, U.S. Pat. No. 3,649,268 and negative-working photosensitive elements are described in Cohen and Fan, U.S. Pat. Nos. 4,174,216 and 4,191,572. The image is developed by toning with a suitable toner which desirably adheres only in the tacky image areas. Excess toner which may be present is removed from the non-tacky image areas to provide, for example, an image which is a proof of an original or which can be used to transfer the image to another surface. Toning provides color flexibility but is associated with environmental concerns because the fine powders are difficult to handle. Also, use of toners results in images having significant relief so that it is difficult to obtain high resolution images when it is necessary to superimpose images to make multicolored images.
Precolored wash-off systems are known in the art. These include precolored diazo based imaging systems such as Van Beusekom, U.S. Pat. No. 3,671,236; Cederburg, U.S. Pat. No. 4,656,114; and Sachi, U.S. Pat. No. 4,666,817 which are developed in a mixture of water and 1-propanol; Krech, U.S. Pat. No. 4,260,673 which is developed in base; and Platzer, U.S. Pat. No. 4,751,166, Mino, U.S. Pat. No. 4,783,390 and Adolphson et al., U.S. Pat. No. 5,075,722 which are developed in water. These systems provide improved resolution. However, since the elements are precolored, color flexibility is limited because preparation of the precolored elements in all the desired colors is not economically feasible. An additional drawback for the systems that are developed in an aqueous base one organic solvent is that they can pose flammability, toxicity, corrosion and/or waste disposal concerns.
Thompson, U.S. Pat. No. 4,737,436 discloses a water based method for making color proof images on a single substrate using a photoresist coating containing a pigment blend. The process provides for the blending of pigments, followed by (a) the combining of the pigment blend with a water soluble photoresist composition, (b) coating a substrate with the photoresist containing the pigment blend, (c) imagewise exposing, and (d) developing to produce an image on the substrate. A second image is then formed on the exposed and developed substrate by forming a second pigment blend, followed by the combining of the second pigment blend with a water soluble photoresist composition, coating the substrate having the first image with the photoresist containing the second pigment blend, imagewise exposing and developing to produce a second image on the substrate.
The process disclosed by Thompson has several disadvantages. Coating solution stability problems have been observed when the pigment blend is combined with photoresist coating. Coating non-uniformity problems can result, with uniform coating becoming increasingly difficult, as the size of the work increases. Since the pigment blend is mixed with the photoresist prior to coating, compatiblity of the pigment with the photoresist coating becomes an issue. This process is also limited in that it does not provide for the application of colorant to the approximate image areas only.
A need exists for a process that generates a high resolution, high quality color image. Further, a need exists for a process that provides color flexibility without the environmental concerns associated with the prior art systems. A need also exists for a process that provides for the application of the colorant to the approximate image areas only.